initialize _engine to NULL

panda/src/display/graphicsStateGuardian.cxx

@@ -230,6 +230,8 @@ GraphicsStateGuardian(CoordinateSystem internal_coordinate_system,
   
   _gamma = 1.0f;
   _texture_quality_override = Texture::QL_default;
+
+  _engine = NULL;
 }
 
 ////////////////////////////////////////////////////////////////////